Electronic Phonons in a Moir\'e Electron Crystal

TL;DR

This paper reports the first direct experimental observation of electronic phonons in a moiré electron crystal within a WS2/WSe2 heterostructure, revealing their properties and controllability, thus advancing understanding of correlated electron phases.

Contribution

It provides the first experimental evidence of electronic phonons in moiré electron crystals, confirming theoretical predictions and demonstrating their tunability via external fields.

Findings

Electronic phonons observed in moiré superlattice

Phonon energies depend on temperature and filling factor

Electronic phonons are tunable by external electric and magnetic fields

Abstract

Collective quantum phenomena, such as the excitation of composite fermions1, spin waves2, and exciton condensation3,4, can emerge in strongly correlated systems like the fractional quantum Hall states5, spin liquids6, or excitonic insulators7. Two-dimensional (2D) moir\'e superlattices have emerged as a powerful platform for exploring such correlated phases and their associated collective excitations8,9. Specifically, electron crystals stabilized by longrange Coulomb interactions may host collective vibrational excitations emerging from electron correlations10, termed electronic phonons, which are fundamentally distinct from atomic lattice phonons. Despite theoretical prediction of their existence in moir\'e electron crystals11, direct experimental evidence has remained elusive. Here we report the observation of electronic phonons in the Mott insulating and stripe phases of a WS2/WSe2 moir\'e superlattice, achieved through light scattering measurements. The phonon energies, temperature and filling factor dependencies, along with theoretical modeling, corroborate their origin as collective vibrations of a correlated electron crystal. Polarization-resolved measurements further indicate rotational symmetry breaking in the Mott state. Notably, these electronic phonons exhibit strong tunability in energy, intensity, and polarization under external electric or magnetic fields, highlighting rich and controllable lattice dynamics of the electron crystal. These findings provide direct spectroscopic evidence for the electronic crystalline nature of correlated phases, opening avenues for probing and manipulating collective excitations in correlated electron systems.